1. Field of the Invention
The present invention relates to an array-type light modulating device which is manufactured by micromachining and which is arranged to change the light transmittance by an electromechanical operation thereof and a method of operating a flat display unit incorporating the array-type light modulating device, and more particularly to a technique for raising the response speed of the array-type light modulating device and the flat display unit.
2. Description of the Related Art
An electromechanical light modulating device has been known which has a structure that thin flexible films manufactured by micromachining are mechanically operated by electrostatic force so that light modulation is performed. As an example of the light modulating device, a structure is known in which thin flexible films each of which is composed of transparent electrodes and a diaphragm are, through support portions, supported by fixed electrodes formed on a light conductive plate.
The foregoing light modulating device is arranged such that a predetermined voltage is applied between the two electrodes so as to generate electrostatic force between the electrodes so that the thin flexible films are deflected toward the fixed electrode. As a result, the optical characteristic of the device is changed to permit light to penetrate the light modulating device. When the applied voltage is made to be zero, the thin flexible film is elastically restored. Thus, the light modulating devices shield light. Thus, light modulation is performed.
When the thin flexible film is deformed or elastically restored by the electrostatic force, the relationship between the applied voltage Vgs and the displacement of the thin flexible film has a hysteresis characteristic. Therefore, also the relationship between the applied voltage Vgs and the light transmittance T has a hysteresis characteristic, as shown in FIG. 17.
With the foregoing hysteresis characteristic, in a state in which a light modulating element is turned off (light is shielded), the turned-off state is maintained when Vgs is not higher than Vth(L). When Vgs is not lower than Vth(H), the turned-on state is maintained. When Vgs is not lower than Vth(H), the light modulation element maintains the turned-on state. When Vgs is not higher than Vs(L), the light modulating element is saturated to the turned-off state. When Vgs has the negative polarity, a positive characteristic is realized which is symmetrical with respect to the axis of ordinate.
A response characteristic of transmitted light is shown in FIG. 18, the characteristic being realized in accordance with the foregoing hysteresis characteristic such that Vs(H) is applied as the applied voltage Vgs in an equilibrium state (a turned-off state) in which no electrostatic stress is not generated in the thin flexible film, followed by making Vgs to be zero after the thin flexible film has sufficiently be deformed.
According to FIG. 18, first transition time xcfx84xcex93 owning to application of the voltage is time caused from electrostatic force (attracting force). Therefore, quick displacement response is realized and also optical response caused from the displacement response is quickly performed. When the applied voltage Vgs is furthermore raised, the response time can be shortened.
On the other hand, the fall time xcfx84f is elastic restoring time which is determined by the material and the shape of the thin flexible film. Therefore, the fall time xcfx84f is slower than the first transition time xcfx84xcex93, in general. As a matter of course, control by dint of the applied voltage cannot be performed.
Therefore, when the light modulating devices are operated in a two-dimensional matrix, scanning time xcfx84 for writing image signals which must be input to the light modulating pixels is undesirably limited to the slower response time. In the foregoing example, scanning time xcfx84 is made to be the fall time xcfx84f. When the scanning time is slow as described above, there arises a problem in that the number of the rows of the matrix cannot be enlarged. When an operating method is employed which realizes the gray scale using time division, another problems arise in that the number of gray-scale levels cannot be enlarged.
A structure having the above-mentioned hysteresis characteristic encounters a fact that a state of the thin flexible film in a state before the writing operation exerts an influence on a next operation. Therefore, to accurately perform the writing operation with satisfactory repeatability, it is preferable that a resetting operation, that is, an equilibrium state (a turned-off state) is realized before the writing operation is performed. Then, the writing operation is performed to realize a required transmittance. If the resetting operation is simply performed before the writing operation, the scanning time for each row, however, is elongated excessively. In this case, the foregoing problem becomes more critical.
It might therefore be feasible to obtain a quick response characteristic by increasing the rigidity of each fluorescent portion of the light modulating device. However, the operating voltage is raised, causing the operating circuit to bear a heavier load. As a result, cost and size reductions are inhibited.
In view of the foregoing, an object of the present invention is to provide an array-type light modulating device and a method of operating a flat display unit with which loss caused from the restoring time can be prevented without deterioration in the image quality and the substantial response time can significantly be shortened if the electromechanical light modulating devices require a long restoring time.
To achieve the foregoing object, according to a first aspect, there is provided a method of operating an array-type light modulating device incorporating electromechanical light modulating devices which are arranged to perform light modulation by using an operation for displacing flexible portions by dint of electrostatic force and an elastic restoring operation of the flexible portions and which are disposed into a two-dimensional matrix configuration, the method of operating an array-type light modulating device comprising the steps of: performing a resetting scan operation which restores the light modulating devices, which is performed for scanning lines except for scanning lines which are reset and which is performed simultaneously with writing scan operation for selecting either of an operation for displacing the devices or an operation for maintaining the present state so that the writing scan operation of each scanning line is continuously performed.
Namely the present invention is characterized in that the resetting scan operation for selected scanning lines is performed within writing scan operation period for scanning lines except for the selected scanning lines.
The foregoing method of operating the array-type light modulating device is arranged to perform the reset scanning operation of the light modulating devices simultaneously with the writing scan operation time for the scanning lines except for the reset scanning line. Therefore, the writing scan for each scanning line can be performed without any loss even if the light modulating devices require a long time to elastically restore the original positions. Therefore, the response time of the array-type light modulating device can significantly be shortened.
A method of operating an array-type light modulating device according to a second aspect is a method, wherein the resetting scan operation for selected scanning lines is performed simultaneously with writing scan operation for scanning lines except for the selected scanning lines so that the writing scan operation of each scanning line is continuously performed.
A method of operating an array-type light modulating device according to a third aspect has a structure that reset scanning time is set to be an integer multiple of writing scan time.
The foregoing method of operating the array-type light modulating device is structured such that the reset scanning time is set to be an integer multiple of the writing scan time. Therefore, the reset scanning time can be elongated by performing a simple change of the design such that a wide degree of design freedom is maintained. If the devices require a long time to elastically restore the original position, the devices can be operated without reduction in the response speed.
A method of operating an array-type light modulating device according to a fourth aspect has a structure that reset scanning operation time is set to be longer than time required for each flexible portion to elastically restore the original position.
The foregoing method of operating the light modulating devices is structured such that the resetting operation is the elastic restoring operation of each of the flexible portions of the light modulating devices. When the reset operation time is set to be longer than the elastic restoring time for the flexible portion, the start of the writing operation is not performed at timing during the elastic restoring operation. Therefore, the operation method enables the elastic restoration to reliably be performed. When the reset operation time is approximated to the elastic restoration time, the writing operation for each device can be performed immediately after the resetting operation. Therefore, the devices can efficiently be operated.
A method of operating an array-type light modulating device according to a fifth aspect has a structure that the elastic restoring operation of each light modulating device is an operation which realizes a light shielded state after the restoration has been completed.
The foregoing method of operating the light modulating device is structured such that the light shielded state is realized after the elastic restoring operation, which is the operation for resetting each light modulating device, has been completed. Therefore, when xe2x80x9cblackxe2x80x9d is, as an image, output in a case where the resetting operation is performed, the light-shielded state is maintained. When xe2x80x9cwhitexe2x80x9d is output, the output is reduced in only the resetting period. However, no critical problem arises. When the resetting operation is performed by realizing the light-transmissible state, output of xe2x80x9cblackxe2x80x9d as the image results in light transmission being caused by the resetting operation. Therefore, the contrast is considerably lowered. As a result, the foregoing lowering of the contrast can be prevented.
A method of operating a flat display unit according to a sixth aspect and incorporating the array-type light modulating device, a flat light source disposed opposite to the array-type light modulating-device and fluorescent members disposed opposite to the flat light source such that the array-type light modulating device is interposed, the method of operating a flat display unit comprising the steps of: operating the array-type light modulating device by an operating method according to any one of the first to fourth aspect; and using light emitted from the array-type light modulating device to cause fluorescent members to emit light to perform display.
The foregoing method of operating the flat display unit, which incorporates the electromechanical array-type light modulating device arranged to complete the resetting operation before the writing operation for each device to short the response time, is structured such that light transmitted through the array-type light modulating device is used to cause the fluorescent members to emit light to perform display. Therefore, the flat display unit can quickly be operated.
A method of operating a flat display according to a seventh aspect has a structure that the flat light source is a light source for emitting ultraviolet rays for exciting the fluorescent members.
The method of operating the flat display unit is able to cause the fluorescent members to be excited to emit light by transmitting or shielding ultraviolet rays emitted from the flat light source by the light modulating devices.